There exists a dispersed power supply system in which a natural energy type power supply system such as a solar cell, or the like, and a non-natural energy type power supply system such as a storage battery, a fuel battery, or the like, are interconnected to the same commercial power grid (hereinafter appropriately abbreviated to a grid) (for example, see PTL 1). Furthermore, a plurality of non-natural energy type power supply systems can be interconnected to the same grid to form a dispersed power supply system. Each power supply system can perform an interconnection operation in which the alternating current power is output through a grid interconnection and an independent operation in which the alternating current power is output independently of the grid in the event of a power failure of the grid.
In the case of a non-natural energy type power supply system, unlike a natural energy type power supply system such as solar power generation, permission from a power company is separately required to sell electric power to the grid. Note that, under the present regulations in Japan, it is specified that the power obtained from a non-natural energy type power supply system should be controlled so as not to be sold. Thus, in order to prevent reverse power flow to the grid (current in the direction of selling power), a current sensor is connected to each power conditioner of a non-natural energy type power supply system to detect current between the power supply system and the grid. In order to prevent selling power to the grid, each power conditioner controls so that a predetermined forward power (current in the direction of buying power) always flows in the current sensor. If the forward power flow is lower than a predetermined threshold (forward power flow threshold) during power supply to a load, each power conditioner decreases its own output to increase the forward power flow from the grid. In addition, if the forward power flow is higher than the forward power flow threshold, each power conditioner increases its own output to decrease the forward power flow from the grid.